Sunscreen preparations, including cosmetics that contain sunscreens, are used to protect human skin and other tissue from the damaging effects of ultraviolet (UV) rays. Among the more effective and successful sunscreen products are those that utilize Titanium Dioxide (TiO2) and Zinc Oxide (ZnO). Others use organic UV absorbers. Some of the more effective of these formulations are visible in use, while others blend with the skin or otherwise become transparent.
The ultraviolet radiation (UVR) spectrum has two distinct regions that are of concern for human health: UVB (290-320 nm) and UVA (320-400 nm). Both UVA and UVB radiation exposure, from the sun or artificial sources are hazardous. UVB causes sunburn and skin cancer in humans. UVA causes skin damage, reduces skin elasticity and induces wrinkles and also contributes to sunburn and cancer. Also, UVA radiation is a year-round phenomenon. UVA radiation passes through window glass and penetrates deeper into the skin than UVB radiation, while UVB radiation is blocked by window glass. High intensities of UVB light are hazardous to the eyes, and exposure can cause welder's flash (photokeratitis or arc eye) and may lead to cataracts, pterygium, and pinguecula formation. A UV attenuating formulation preferably includes both a UVA and a UVB blocking constituents (“broad spectrum” coverage) to prevent most of the UV radiation within the range of about 290-400 nm from reaching human skin, hair or other organs and tissues. There is also so called UVC radiation within 100-290 nm wavelength range, the strongest and potentially most harmful form. While UVC produced by the sun is almost entirely absorbed by the earth's ozone layer and is therefor not usually considered a health concern below the ozone layer, it should be noted that the UVC radiation can be found in mercury arc lamps, germicidal lamps and other sources.
Currently both organic and inorganic sunscreens are commercially available. To satisfy the “broad spectrum” attenuation criteria in many formulations it is common practice to utilize two or more active ingredients with complimentary absorbance spectra since typical organic or inorganic sunscreens block only a portion of the total UVR spectra. When organic sunscreen preparations are used, issues of photo-induced and non-photo induced toxicity and allergy have been observed due to long-term use of organic sunscreens and are becoming increasingly of concern.
Among the organic UV radiation absorbing ingredients of UVB filters that are used in commercial sunscreen formulations in the U.S.A. are paramethoxycinnamic acid esters, such as 2-ethylhexyl paramethoxycinnamate, commonly referred to as octyl methoxycinnamate or PARSOL MCX, octyl salicylate, and oxybenzone. The common organic UVA filters used in commercial sunscreen formulations are the dibenzoylmethane derivatives, particularly 4(1,1-dimethylethyl)-4′-methoxydibenzoylmethane, which is also called avobenzone and sold under the name PARSOL 1789. Other dibenzoylmethane derivatives and other organic compounds can also be used as UVA filters.
The above described UVA filters can rapidly degrade, when used alone or when combined with the UVB filters. Typically, the UVB filters are combined with the UVA filters in an oil-based formulation. This oil solution or “oil phase”, as it referred to by formulators of cosmetic products and sunscreens, is then mixed with an aqueous solution to make an emulsion, which becomes the cream or lotion form of a sunscreen or cosmetic. There can be a situation when one photoactive compound in a sunscreen formulation promotes photodegradation of another photoactive compound in the composition. For example, when avobenzone is combined with octyl methoxycinnamate, rapid photodegradation of both the dibenzoylmethane derivative and the octyl methoxycinnamate is observed.
The most common inorganic sunscreen agents include ZnO and TiO2 particles. They can be suspended either in oil or water based media, as opposed to organic sunscreens agents that are mostly soluble in oil-based media used in cosmetic formulations. By appearance, both ZnO and TiO2 are white pigments with TiO2 demonstrating more visible “whiteness” than ZnO. The degree of perceived transparency depends on many factors including how much of sunscreen formulation is applied to the skin (“thickness”) and the concentration of the particles in the formulation. In formulations with high SPF factors, these agents are quite visible on the skin, which may be undesirable for some users and for use over large areas of the body.
When used in combination, ZnO and TiO2 are generally considered complimentary and provide higher degrees of UVA and UVB protection than when used alone. However, preparation of the mixtures of these particulates requires special processing (for example, separate dispersion of the ZnO in the oil phase and the TiO2 initially in the water phase. There is also currently a concern that when ZnO and TiO2 are formulated with smaller particle sizes, the rate at which harmful hydroxyl radical generation increases due to the inherent photoactivity of these materials.
UV radiation can also degrade cosmetic formulations and cosmetic products, such as, for example, the structural component such as the polymer as well as the color agents such as pigments or dyes. This photo-induced degradation can lead to color fading and deterioration effects which are undesirable in the cosmetic product.